Process for producing copolymer latex

Abstract

A process for producing a copolymer latex for use as a binder in a paper coating composition by emulsion polymerization while monomers and a molecular weight modifier are continuously introduced into a reactor, characterized in that (a) the molecular weight modifier is used in an amount of X o parts by weight (in which O<X o ≦10) per 100 parts by weight of a monomeric mixture composed of (1) 20 to 50% by weight of an aliphatic conjugated diene monomer, (2) 15 to 70% by weight of a vinyl aromatic monomer, (3) 5 to 40% by weight of an alkyl ester of an ethlenically unsaturated carboxylic acid, (4) 0.5 to 10% by weight of an ethlenically unsaturated acid monomer, and (5) 0 to 30% by weight of a vinyl cyanide monomer, and (b) the molecular weight modifier is continuously added so that the amount of the molecular weight modifier added per unit time at any time t in a time period of adding the monomers and the molecular weight modifier is X(t) parts by weight per 100 parts by weight of the monomeric mixture added per unit time, wherein, (i) X(t) takes a minimum value of Xmin parts by weight and a maximum value of Xmax parts by weight within said time period of addition, and (ii) O≦Xmin<X o , and Xmax<1.2X o .

What is claimed is: 
     
       1. A process for producing a copolymer latex for use as a binder in a paper coating composition which comprises continuously introducing into a reaction zone under emulsion polymerization conditions (a) a monomeric mixture composed of (1) 20 to 50% by weight of an aliphatic conjugated diene monomer, (2) 15 to 70% by weight of a vinyl aromatic monomer, (3) 5 to 40% by weight of an alkyl ester of an ethylenically unsaturated carboxylic acid, (4) 0.5 to 10% by weight of an ethylenically unsaturated acid monomer, and (5) 0 to 30% by weight of a vinyl cyanide monomer, in the presence of   (b) a molecular weight modifier in an amount of X o  parts by weight, per 100 parts by weight of the monomeric mixture (a), where 0<X o  ≦10, wherein the molecular weight modifier is added to the monomeric mixture in the reaction zone in continuously varying amounts such that the amount X(t), in parts by weight, per 100 parts by weight of the monomer mixture, of the molecular weight modifier added per unit time at any time t during the introduction to the reaction zone of the monomeric mixture and molecular weight modifier has both a minimum value Xmin parts by weight and a maximum value Xmax parts by weight with 0≦Xmin<X o  and Xmax≧1.2X o .   
     
     
       2. The process of claim 1 wherein the amount X(t) of the molecular weight modifier added per unit time is increased as the addition time lapses, so that it is minimum at the start of addition and becomes maximum at the end of addition. 
     
     
       3. The process of claim 1 wherein the amount X(t) of the molecular weight modifier per unit time is decreased as the addition time lapses, so that it is maximum at the start of addition and becomes minimum at the end of addition. 
     
     
       4. The process of claim 1 wherein said time period of addition includes at least one time zone where the amount X(t) of the molecular weight modifier per unit time increases as the addition time lapses, and at least one time zone where the amount X(t) of the molecular weight modifier added per unit time decreases as the addition time lapses. 
     
     
       5. A process for producing a copolymer latex combining good print gloss, adhesion strength and blister resistance during offset printing when used as a binder in a paper coating composition, said process comprising continuously introducing into a reaction zone under emulsion polymerization conditions a monomer mixture comprising (1) 25 to 40% by weight of an aliphatic conjugated diene monomer, (2) 30 to 60% by weight of a vinyl aromatic monomer, (3) 5 to 25% by weight of an alkyl ester of an ethylenically unsaturated carboxylic acid, (4) 1 to 5% by weight of an ethylenically unsaturated acid monomer, and (5) 0 to 30% by weight of a vinyl cyanide monomer, in the presence of a molecular weight modifier in an amount of X o  parts by weight, per 100 parts by weight of the monomeric mixture, where 0<X o  ≦10, wherein the molecular weight modifier is added to the monomer mixture in the reaction zone in continuously varying amounts such that the amount X(t), in parts by weight, per 100 parts by weight of the monomer mixture, of the molecular weight modifier added per unit time at any time t during the introduction to the reaction zone of the monomer mixture and molecular weight modifier has both a minimum value X min  parts by weight and a maximum value X max  parts by weight with 0≦X min  <X o , and X max  ≧1.2X o .   
     
     
       6. A process for producing a copolymer latex combining good print gloss, adhesion strength and blister resistance during offset printing when used as a binder in a paper coating composition, said process comprising continuously introducing into a reaction zone under emulsion polymerization conditions a monomer mixture comprising (1) 20 to 50% by weight of 1,3-butadiene, 2-methyl-1,3-butadiene or 2-chloro-1,3-butadiene, (2) 15 to 75% by weight of styrene, α-methylstyrene or vinyltoluene, (3) 5 to 40% by weight of methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, 2-ethylhexyl acrylate, monoethyl fumarate, diethyl fumarate, monoethyl itaconate, dimethylitaconate, monobutyl fumarate, or monopropyl maleate, (4) 0.5 to 10% by weight of acrylic acid, methacrylic acid, crotonic acid, cinnamic acid, itaconic acid, fumaric acid or maleic acid, and (5) 0 to 30% by weight of acrylonitrile, alpha-chloroacrylonitrile, methacrylonitrile or alpha-ethylacrylonitrile, in the presence of a molecular weight modifier in an amount of X o  parts by weight, per 100 parts by weight of the monomeric mixture, where 0<X o  ≦10, wherein the molecular weight modifier is added to the monomer mixture in the reaction zone in continuously varying amounts such that the amount X(t), in parts by weight, per 100 parts by weight of the monomer mixture, of the molecular weight modifier added per unit time at any time t during the introduction to the reaction zone of the monomer mixture and molecular weight modifier has both a minimum value X min  parts by weight and a maximum value X max  parts by weight with 0≦X min  <X o , and X max  ≧1.2X o .   
     
     
       7. The process of claim 6 wherein the molecular weight modifier is at least one compound selected from the group consisting of mercaptans, sulfides and alkyl halides. 
     
     
       8. The process of claim 5 wherein the molecular weight modifier is at least one compound selected from the group consisting of mercaptans, sulfides and allkyl halides. 
     
     
       9. The process of claim 1 wherein the molecular weight modifier is at least one compound selected from the group consisting of mercaptans, sulfides and alkyl halides.